In the process of machining metals, as in the practice of milling, rotary tables are extensively used to securely hold the part to be milled while a rotary motion is being imparted to the work holding table itself. The angular velocity of these tables is usually very low. For a table having a diameter of about thirty inches, the maximum velocity is about four or five revolutions a minute. This rotational speed results in a rim velocity of about four or five hundred inches per minute which agrees rather well with the rapid traverse rates of speeds of milling machines linear axis slides.
Most rotary table drives use electric motors or even hydraulic motors as their primary sources of rotary motion. Since these motors are high speed devices, it normally requires some type of speed reduction unit to obtain the five revolutions per minute speed at the table. These reductions are usually gear transmissions having a reduction ratio of as much as 400 to 1. At the present state of the art, these tables are called upon to be able to make some extremely small and accurate angular movements. The size of these movements is measured in seconds of arc. A full circle contains 360 degrees of 1,296,000 seconds of arc. The usual resolution used for angular movements is one-thousandth of a degree which is 3.6 seconds of arc. When one considers the size of this small angular movement and also the great ratio of speed reduction, it becomes apparent that the smallest amount of backlash in the gear reducer transmission will greatly exceed the minimum resolution, 3.6 seconds of arc. In order to remedy this difficulty, it has been the practice to devise and build backlash elimination devices in the gear drive mechanisms. These devices are expensive, difficult to keep in adjustment and usually low in mechanical efficiency.
In the rotary designs mentioned above, it is necessary to have some device which is capable of measuring very small angles and here again, because of the backlash problem in the transmission, it is necessary to have the measuring device mounted co-axially on the table center in order that all transmission causing backlash be avoided. This calls for the measuring device to have a resolution of 3.6 seconds of arc or better. At the present time, there exist only a very few angle measuring devices that can possibly be used to measure such small angles. All are very fragile and difficult to keep in adjustment. Measuring devices of this nature are also quite expensive.